1. Field of the Invention
The present invention relates to optical encoding systems. In particular, the present invention relates to an optical encoder having a light source bias circuit which adjusts the electrical bias to the light source to compensate for aging and temperature effects thereon.
2. Description of the Prior Art
Optical encoders are often used to "observe" mechanical motion or activity and convert this "observation" into an electrical signal which may be used to represent or characterize such motion or activity. A simplified pictorial illustration of an optical encoder is shown in FIG. 1. A basic optical encoder 10 includes a light source 12, an optical path 14 along which light 16 from the light source 12 travels, and a light receiver 18. As described more fully below for FIG. 2, the light receiver 18 produces an electrical signal representative of how much light 28 it is receiving from the light source 12 via the optical path 14. Typically, a code disk 20 is positioned on a shaft 22 for rotation across the optical path 14. A radial ring of alternating opaque 24 and transparent 26 sections are formed near the outer edge of the code disk 20. The alternating opaque 24 and transparent 26 sections modulate the intensity of the light 16 traveling along the optical path 14, and therefore modulate the light 28 reaching the light receiver 18, as the code disk 20 rotates across the optical path 14. This causes the electrical signal produced by the light receiver 18 to change accordingly.
In some encoding systems this electrical signal may represent a series of binary bits. These are known as incremental encoders. In other encoding systems, known as absolute encoders, multibit binary "position" words formed from signals from multiple tracks of indicia are used to identify each resolvable position of the encoder. Still other encoding systems employ tracks of indicia which produce sinusoidal or other analog signals which are then converted into binary position words.
In all cases, but especially for analog encoders, it is important that the relative range of received light 28 and the absolute amounts of light within that range remain consistent. Any variations in such a relationship will introduce errors in the resulting position information. This means that the light source 12 must consistently put out substantially the same amount of light 16. This will ensure that any variations in the light 28 received at the light receiver 18 will be those produced by the indicia 24, 26 on the code disk 20 only, thereby making the electrical signal produced by the light receiver 18 truly representative of such indicia-imparted 24, 26 modulation.
Therefore, the encoding system 10 includes monitoring within the light receiver 18 to determine whether the light source 12 is consistently emitting the desired amount of light 16. This monitoring is done by measuring the light 28 received by the light receiver 18. A feedback signal based upon this measurement is provided to the light source 12 so as to effect any biasing adjustments which may be required to cause the light source output 16 to remain within the desired range.
Typically, to reliably monitor the light 28 received by the light receiver 18 an unobstructed light path, i.e., a light path free of any modulation, is desirable. Therefore, a radial transparent ring 27 is formed inboard from the aforementioned ring of opaque 24 and transparent 26 sections. This transparent ring 27 provides an unmodulated light path for reliably monitoring the light 28 received by the light receiver 18.
As explained more fully below for FIG. 2, in the past this monitoring has been accomplished by the use of multiple light sensors 44, 45, 46 within the light receiver 18. One or more light sensors 44, 45 are used as described above for sensing the modulation of the light 16 within the optical path 14 by the rotating ring of opaque 24 and transparent 26 sections and generating an electrical signal proportional thereto. Another light sensor 46 is used to monitor the total light 28 received from the light source 12 via the transparent ring 27 and provide the feedback signal for adjustment thereof. However, this arrangement requires a correlation or calibration between the light sensors 44, 45, 46 with respect to their respective sensitivities to light. Typically, the light source 12 is a light emitting diode ("LED") and the light sensors 44, 45, 46 are photosensitive diodes ("photodiodes"). A block diagram of a simple example of such a system 40 is illustrated in FIG. 2.
The basic components of a typical, simple optical encoding system 40 include: a LED 42; three photodiodes 44, 45, 46; an error amplifier 48; a resistor 50; two buffer amplifiers 52, 53; and an output amplifier 64. The LED 42, error amplifier 48 and resistor 50 make up the light source 12. The photodiodes 44, 45, 46, buffer amplifiers 52, 53 and output amplifier 64 make up the light receiver 18. (The optical path 14 may be any defined optical path between the light source 12 and light receiver 18.)
The "data" photodiodes 44, 45, buffer amplifiers 52, 53 and output amplifier 64 generate the data output signal which is a function of the amount of light 56, 57 incident on the photodiodes 44, 45. In turn, the amount of this incident light 56, 57 is a function of the amount of optical modulation within the optical path 14 induced by the rotating ring of opaque 24 and transparent 26 disk sections. The "monitor" photodiode 46 monitors the light 58 received from the LED 42 via the transparent ring 27 and generates the feedback signal 62 necessary to cause the total light output 54 from the LED 42 to remain consistent.
The LED 42 is driven into a light emitting state by the output of the error amplifier 48, receiving its supply current from the power supply V.sub.s. The error amplifier 48 is a "transconductance" amplifier, providing an output current 63 dependent upon the relative magnitudes of its input voltages V.sub.REF, V.sub.F (discussed below). According to means well known in the art, its output current 63 is current-limited (e.g., 35 milliamperes maximum) so as to prevent overdriving the LED 42 upon initial power up of the system 40.
The light 54 emitted by the LED 42 produces light 56, 57, 58 which is received by the photodiodes 44, 45, 46. The data photodiodes 44, 45 generate data signals 60, 61 in the form of electrical currents which are buffered by buffer amplifiers 52, 53 and compared by an output amplifier 64 to create the final data output signal. The buffer amplifiers 52, 53 are "transimpedance" amplifiers, providing output voltages 66, 67 dependent upon their respective input currents 60, 61, namely the currents flowing through their respective photodiodes 44, 45. The output amplifier 64 is a voltage comparator when a binary data output signal is desired, or a differential amplifier when an analog data output signal is desired. Voltage comparators and differential amplifiers are both well known in the art.
The monitor photodiode 46 generates a feedback signal 62 in the form of an electrical current which in turn generates a feedback voltage "V.sub.F " across the resistor 50. This feedback voltage V.sub.F is compared to a reference voltage "V.sub.REF " by the error amplifier 48. The difference between the feedback V.sub.F and reference V.sub.REF voltages determines whether or not the current drive 63 for the LED 42 is to be increased or decreased, thereby causing the LED 42 to emit more or less light 54.
As will be recognized by one of ordinary skill in the art, the two data photodiodes 44, 45 of FIGS. 1 and 2 constitute the simplest example of a photodiode array. By using a photodiode array rather than a single photodiode, phase information (e.g., information regarding the relative angular orientation of the disk 20) may be quantified by the data output signal. In a simple array as illustrated in FIG. 1 where the sizes of the opaque 24 and transparent 26 disk sections are substantially equal to the sizes of the data photodiodes 44, 45, the data photodiodes 44, 45 are physically situated so that when a transparent section 26 fully covers the first photodiode 44, an opaque section 24 fully covers the second photodiode 45, and vice versa. In this situation, the first photodiode 44 behind the transparent section 26 receives maximal light 56 and generates a maximal first data signal 60. The second photodiode 45 behind the opaque section 24 receives minimal light 57 and generates a minimal second data signal 61.
As will be appreciated by one of ordinary skill in the art, as the disk 20 rotates the orientations of the opaque 24 and transparent 26 sections shift, periodically resulting in the first 44 and second 45 photodiodes producing a minimal first data signal 60 and maximal second data signal 61, respectively, the inverse of those described above. As rotation of the disk 20 continues, the aforementioned changes in the data signals 60, 61 continue between minimal and maximal signal magnitudes, with relative increases in the first signal 60 being substantially equal to and corresponding with relative decreases in the second signal 61, and vice versa. Thus, between the extremes where only the first photodiode 44 and then the second 45 is illuminated, both photodiodes 44, 45 are partially illuminated. By proper processing of their respective data signals 60, 61 (the sum of which is substantially constant), the relative angular orientation of the disk 20 may be determined.
As will be appreciated by one of ordinary skill in the art, use of larger photodiode arrays (i.e., more than two data photodiodes 44, 45) and/or "phase plates" (i.e., smaller, more precisely dimensioned and positioned slits placed within the optical path 14 to more precisely define the optical path 14) will increase resolution of the phase information. Use of larger photodiode arrays and/or phase plates allows detection of smaller change increments in the relative angular orientation of the disk 20.
Since all of the photodiodes 44, 45, 46 are generating electrical signals based upon the amount of respective light 56, 57, 58 received by them, albeit for different purposes, it is important that their output signals 60, 61, 62 vary in a like manner, or at the very least, that any differences in the way they respond to the same light conditions be identified and compensated. This requires that the manufacturing processes by which these photodiodes 44, 45, 46 are manufactured produce photodiodes whose performance characteristics are virtually identical, or at least consistent with respect to their differences.
Furthermore, photodiodes are relatively large compared to other electronic devices. The main components of an optical encoder are typically integrated onto a single integrated circuit chip. Therefore, the need for a separate monitor photodiode 46 requires dedication of a relatively large circuit chip area (e.g., approximately 1,000 square mils per photodiode) within the integrated circuit chip.
Accordingly, it would be desirable to have an optical encoder design which eliminates the need for a separate monitor photodiode, thereby eliminating the need for photodiodes having matched or correlated characteristics. It would be further desirable to eliminate the need for a separate monitor photodiode so as to eliminate the need for the large surface area required therefor when integrating the encoder design onto a single integrated circuit chip.